Writing as background for their study, Waryszak et al. (2018) say that there has been "surprisingly little attention given to how the chemical control of weeds may be altered under future climate conditions." In this regard, they note that they are aware "of only 21 weed species that have been examined for changes in herbicide tolerance under elevated CO2," which represents the smallest fraction of the thousands of weed species that exist globally. Hoping to remedy this data shortfall, the team of four Australian researchers examined the growth and survival response of fourteen common weed species to two herbicides (glyphosate and fluroxypyr-meptyl) under ambient (380 ppm) and elevated (550 ppm during daylight hours only) CO2 conditions.

Their experiment was conducted in climate-controlled glasshouses, where the weeds were grown in pots with adequate water and nutrients. Herbicide treatments were then applied during the active growing phase of the plants (ranging from 45-121 days of growth) at recommended and twice-recommended doses. Plant biomass was examined just prior to herbicide application and survival rates were monitored following herbicide treatment.

In discussing their findings Waryszak et al. report that plant dry weights were greater under elevated CO2 in 11 of the 14 weed species, yet the increase was significant in only one of the plants studied. In contrast, two of the weed plants produced more biomass under ambient CO2.

With respect to plant survival after applying the recommended dose of herbicide, only two of the 14 weed species exhibited increased herbicide tolerance under elevated CO2, which number dropped to one in the elevated CO2 and double herbicide application treatment. In contrast, one weed specie experienced decreased tolerance/survival in elevated CO2 conditions. Consequently, given the above findings, Waryszak et al. conclude that "a universal trend towards greater herbicide tolerance under future CO2 conditions is not likely." Thus, it would appear that not only does elevated CO2 tend to not enhance weed biomass, it does not appear to alter herbicide effectiveness either. And that is good news for those concerned about the future management and control of weeds.